This study's purpose is to establish the mechanism(s) of progression of communicating syringomyelia. Communicating syringomyelia usually accompanies abnormalities at the craniocervical junction. Measurement of intraventricular pressure, intrathecal pressure, and intrasyrinx pressure provides data that elucidate the hydrodynamic mechanism(s) of progression of syringomyelia. Radiographic testing, including magnetic resonance imaging (MRI) flow studies and ultrasonography, demonstrates how pathologic anatomy alters normal cerebrospinal fluid (CSF) flow. The effect of posterior fossa craniectomy, upper cervical laminectomy, and duraplasty on CSF and pressure, syrinx size, and neurological function is being evaluated. Twelve patients have been treated. Only one patient had communication between the fourth ventricle and the syrinx. Despite obstruction of CSF pathways at the foramen magnum, phase and cine- MRI demonstrated pulsatile syrinx and cervical subarachnoid CSF flow. Ultrasonographic measurements demonstrated tonsillar descent, cord and syrinx constriction, and caudal syrinx fluid flow during systole. CSF pressure measurements showed that intracranial pulse pressure was transmitted well to the cervical subarachnoid space and syrinx but poorly to the lumbar thecal space. Because intracranial pressure is transmitted despite obstruction of the subarachnoid space at the foramen magnum, we conclude that the cerebellar tonsils and the brainstem act on a partially enclosed spinal subarachnoid space to generate cervical subarachnoid CSF pressure waves. These waves compress the spinal cord from without-not from within as has previously thought-to propel the syrinx fluid downward with each heartbeat. Syrinx progression occurs as a consequence. CraniocervicaI decompression and duraplasty improved CSF flow at the foramen magnum in all patients. All syrinxes decreased in size after surgery. All pressure measurements were performed without complication, including postoperative measurement of cervical and lumbar pressure.